Linear sweep frequency generator with sampling circuit phase control loop

ABSTRACT

The invention includes means to generate a substantially linear FM waveform pulse. A phase comparison of the FM waveform pulse is made with respect to several reference constant frequency waveform signals whose frequencies and phase are made to match an ideal FM waveform pulse at preselected points during the pulse. The frequencies of the reference signals are chosen to be multiples of a reference oscillator. The frequency of the oscillator is chosen to give the desired number of correction points during the pulse. The FM waveform pulse and the reference signals are mixed in separate mixers and the output of each mixer is sampled at a predetermined time by a gate. The outputs of the gates are then integrated and used to correct the FM waveform pulse over the time interval just prior to the sample time. This system will reach a stable equilibrium when the phase of the reference signals and the FM waveform pulse are in phase quadrature since the output of all the mixers are zero for this condition. When a sufficient number of samples are taken, the FM waveform pulse at equilibrium is a highly linear signal.

United States Patent Lloyd R. Blair; Gr g y 11. Martin, both of Phoenix,Arlz. [21] Appl. No. 884,927

[72] lnventors [22] Filed Dee. 115, 1969 [45] Patented Nov. to, 1971[73] Assignee Goodyear Aerospace Corporation Arkon, Ohio [54] LINEARSWEEP FREQUENCY GENERATOR WITI-ll SAMPLING CIRCUIT PHASE CONTROL LOOP[56] References Cited UNITED STATES PATENTS 3,221,266 11/1965 Vitkovits,.lr. 331/4 X 3,382,460 5/1968 Blitz et a] 331/4X 11 1 MiLM PrimaryExaminer-Roy Lake Assistant Examiner-Siegried H. Grimm Attorney-l. G.Pere ABSTRACT: The invention includes means to generate a sub stantiallylinear FM waveform pulse. A phase comparison of the FM waveform pulse ismade with respect to several reference constant frequency waveformsignals whose frequencies and phase are made to match an ideal FMwaveform pulse at preselected points during the pulse. The frequenciesof the reference signals are chosen to be multiples of a referenceoscillator. The frequency of the oscillator is chosen to give thedesired number of correction points during the pulse. The FM waveformpulse and the reference signals are mixed in separate mixers and theoutput of each mixer is sampled at a predetermined time by a gate. Theoutputs of the gates are then integrated and used to correct the FMwaveform pulse over the time interval just prior to the sample time.This system will reach a stable equilibrium when the phase of thereference signals and the FM waveform pulse are in phase quadraturesince the output of'all the mixers are zero for this condition. When asufiicient number of samples are taken, the FM waveform pulse atequilibrium is a highly linear signal.

GATED PRF TRIGGER IN ILLATOR OUTPUT SWEPT I2 OSCILLATOR SHIFT REGISTERTIME COMPARATOR DETECTOR LOW PASS FILTER GATE GATE GATE INT. INT,

INT

GATE GATE GATE PATENTED T5197! 3.621.450

SHEET 1 BF 2 MI TRANSMITTED E i SIGNAL l l 4 F RECEIVED SIGNAL 5 i A;[167i 3 I L U1 5% l REFERENCE 1c SIGNAL I i AFTER-MIXING A-F SIGNAL 156-2 AMPLITUDE 54 E 53 Hi f FREQUENCY OF REFERENCE k m OSCILLATORINVENTORS o LLOYD R. BLAIR 1%, t t GREGORY L. MARTIN TIME MmrwmATTORNEYS PAIENIEDuuv 16 Ian SHEET 2 OF 2 W OI INVENTORS LLOYD R. BLAIRGREGORY IL. MARTIN mmhmamm .rhzIm p i- 0 W56 P56 P20 v 0v mm mm PamBmEmil-H. wm a 30 mwx.

PDmPDO mokuwhwo EOCEESOQ MEI.

Maw

2 mwwwEh Em ATTORNEYS LINEAR SWEEP FREQUENCY GENERATOR WITH SAWWG QMCUMTPHASE CONTROL LOOP Many radar designs require that a linear frequencymodulated waveform be transmitted. in some cases the requirement arisesbecause of peak power limitations in the transmitter. in other cases itis desired to use FM radar techniques for range discrimination oftargets, or similarly it may be desirable to use the stretch techniqueto provide a reduction in bandwidth of the video signal. in most casesit is desirable to keep the phase errors in the transmitted signal assmall as possible so that range resolution is not degraded.

The primary purpose of the invention is to provide a technique forgenerating an FM pulse with a dispersion factor as large as or more.

For a better understanding of the invention reference should be made tothe accompanying drawings wherein FIG. 1 represents a plurality ofgraphs indicating frequency versus time plot of various radar signals;

FIG. 2 is a graphic illustration of the spectrum of a target aftermixing;

FIG. 3 is a graph indicating the signals used in phase correcting achirp generator; and

H6. 4 is a schematic block diagram of a phase-corrected chirp system.

To establish a need for such a dispersed pulse one may assume that it isdesired to transmit a wide band radar pulse to obtain fine rangeresolution and still be able to record the return video signal usingexisting relatively narrow band recorders. Such requirement can be metby transmitting a linear FM signal and mixing the return signal with oneor more signals whose FM rate is the same as the transmitted pulse. Thisoperation may be recognized as the well-known FM radar method ofobtaining range resolution.

A relationship between various parameters of such a FM system may beobtained by referring to FIG. 1. in FIG. 1, the transmitted signal is alinear FM signal which is swept through a frequency band 13, in a timeperiod T, as shown in Graph A. The return signal characteristics are Sand S as shown in Graph B, where the targets consist of two pointreflectors separated by a slant range interval which corresponds to atime differential At. The reference signal which is shown in Graph C hasthe same FM rate as the signals in Graphs A and B. After mixing, signalsS and 8 are converted to two constant frequency signals separated by afrequency interval Af, as shown in Graph D.

From the geometry of Graph B, it is apparent that:

To determine system resolution reference should be made to FIG. 2, wherethe spectrum of a target signal after mixing is plotted. Two targetswill be near the point of limiting resolution if they are separated by afrequency interval:

filaitF T Thus, it is seen that the limiting time resolution as obtainedfrom equations 1 and 2 above is which as may be expected, is inagreement with what would be expected of a radar with a bandwidth B.

The range interval which can be recorded in a single chan nel of therecorder is limited by the bandwidth of the recorder f Thus lettingAf=f,, we find that a slant range swath width per channel of At Tfl/B(4) is obtained.

To obtain the greatest possible recording interval per channel, it isnecessary to transmit as long a pulse as other constraints will allow.The number of recording channels which must be operating simultaneouslyis =R F i I f, where T is the swath time interval which is set by theswath width.

This expression is valid for T T.

Since T B, and f are determined by system and component specifications,the invention contemplates that T must be made very large to minimizethe number of channels. Thus, assuming range resolution requirements of2 feet then B 500 Me. If the radar pulse repetition frequency and swathwidth allow the transmitted pulse to be 50 microseconds long it isnecessary to obtain a dispersion factor of GENERATION OF THEDllSlPlERSED PULSE It is an important feature of the invention togenerate the highly dispersed pulse with phase errors no greater thanplus or minus 1r/2 radians in order to effectively utilize the bandwidthwhen the pulse is compressed. The invention contemplates that aphase-corrected chirp utilizing a phase control system which is easy toinstrument for wide band chirp waveforms will be utilized. A phasecomparison of the FM waveform is made with respect to several constantfrequency waveforms whose frequencies and phase are made to match anideal FM waveform at preselected points during the pulse.

The frequencies of the reference signals are chosen to be multiples of asingle oscillator as shown by the Graph of Fig. 3. The frequency of theoscillator is chosen to give the desired number of correction pointsduring the pulse. Certain conditions must be satisfied so that the phaseof the reference signals is the desired value at the time that thefrequency of the swept signal matches the frequency of the reference.Specifically, let the phase of the swept signal be where f is the FMrate of the signal.

The phase of the reference is 0=21rjj,t

where j], is the frequency of the reference. To ensure that the sweptsignal changes frequency by an amount j}, between sample times letfi'=fo (9) The time interval between samples is 1. Therefore, theharmonies of 1;, will match the FM signal frequency at the sample t5 pomr=1,,=m (10) Let . Fd/fl, where d is an integer so that referencestraverse an integral number of cycles between samples. Therefore, fromequations (9) and l 1 the required reference frequency flit f,

is obtained. Equation 1 1 may be written as have (fn /fo Upon combiningequations l 2) and 15) further we have 0 =rm d Therefore, by requiringthat d be an even integer, such as 32. 6,, is a multiple of 21r asdesired.

A block diagram of a mechanization of the phase-correcting system isshown in FlG. 4. The actual structural components illustrated in H0. 4comprise a gated oscillator 10, a swept oscillator 12, a shift register14, a ramp generator 116, a lowpass filter 18, a detector 20, and a timecomparator 22. It may be noted that the swept oscillator i2 operates inthe microwave region and its signal is mixed with a stale 24 in mixer 26to obtain the waveform described by equation (7 The swept signal and thereferences after passing through multiplier X through X, are mixed inappropriate mixers 28 through 34 and the output of each mixer is sampledat the proper time by a respective gate 36 through 42. The gates 36through 42 are activated by a ring counter in the shift register 14which is in turn triggered at a rate fJ32 by the divide by 32 signal 44as actuated by oscillator 10.

The outputs of the gates 36 through 42 are then passed to respectiveintegrators 44 through 50 and used to correct this sweep rate over thetime interval just prior to the sample time. This is accomplished by theoutputs of the integrators passing to respective gates 52 through 58with the gates being appropriately controlled by a signal from shiftregister 14 and the gate outputs being sent to ramp generator 16. Thesystem will reach a stable equilibrium when the phase of the referenceand the swept signal are in phase quadrature since the output of themixer 26 is zero for this condition. When a sufficient number of samplesare taken, the swept signal at equilibrium is a highly linear FM ramp.

The width of the sample gates 36, through 42 are selected so that thephase shift in the signal after mixing does not change by more than 45during the sample time. The phase of I the error signal near thezerobeat time 1,, is

For the case B=500 Mc and T=50 asec. and j'-l0 Mc/psecon d, t,.= t0.l76sec. or the sample time can be 0.35 psec. in duration.

The number of samples which must be made depends upon the accuracy ofthe uncorrected waveform. Naturally, this accuracy depends on thecharacteristics of the swept oscillator 12. The primary error inmatching the swept waveform to the desired waveform is due to anincorrect F M rate. The number of samples is critical and is chosen sothat the phase error of the uncorrected input signal waveform does notexceed 90 with respect to an ideal FM waveform between samples. If thephase error of 1r/2 radians is accumulated over a time interval then theallowable error in the FM rate is im/ 27) (19) This equation is derivedby differentiating equation (17) with respect toand letting Thedispersion over a time interval 1' and corresponding frequency interval)1, is

=rfl =f (2 By combining equations (l9) and (20), the allowablefractional error in the FM rate is found to yield The fractional errormay be readily held to 1.5 percent and therefore a dispersion of 32 maybe attained with phase error no greater than 1r/2. Taking d==32 and F"Mc/psec. we find that r=l.79;tsec. Thus, a correction would be madeevery 1.79 useconds, approximately.

Therefore 28 correction points are required during the 50 psecond pulsefor the above assumptions. The frequency of the fundamental referenceoscillator is thus found from the q a on.

' f1=fr to be 17.9 Mc.

LOCK-ON AND ERROR CHECK SYSTEM The output signals from one or more ofthe mixers can be used in a manner which will unambiguously sensefrequency errors in the FM waveform. This feature can be used as an aidduring lock-on or as a test feature to detect a malfunction andthereupon initiate a new lock-on.

As shown in FIG. 4, the output of a mixer. such as mixer 34, may bepassed through the low-pals filter 18 and by detecting the output indetector 20. the zero beat-time can be deterif a simple low-pass filterof optimum time constant is used the detected pulse width isapproximately If a pulse compression network is used over oneintersample period 1-, then the detected pulse width is approximatelyTaking the caseFlO Mc/p.sec., and 1=1.79 psee, we have AT =5Xl0""'sec.approximately and AT SXIU' sec. approximately.

The entire pulse is assumed to be 50 psec. in duration which thereforemeans the technique described above provides a timing accuracy to 0. 1percent. In practice, the timing accuracy can be improved beyond theabove values by a factor of 10 or so since the leading edge of the pulsemay be used as a timing reference. The detection system would be madeindependent of phase by the use of inphase and quadrature channels.

The lock-on process consists of a sequence of several events. First, theswept oscillator 12 is phase-locked to the microwave stalo 24 just priorto each trigger pulse 60. The phase-locking is accomplished bycontrolling the base line level of the ramp from the generator 16. Thephase lock loop is disabled during the sweeping period.

The FM rate is now set to within 1 percent or so of the desired value byuse of the zero beat-time comparator 22. After a few pulse periods thefirst phase error integrator 44 is activated and its output is used tocorrect the ramp slope in generator 16 so that the phase of the sweptsignal is correct at time The second phase error integrator 54 isactivated after number 52 has reached equilibrium. This integratorcorrects the ramp slope after time t,.

Also, the zero beat-time comparator 22 is operated on an open loop basisafter the first phase error integrator 44 takes control. The curvatureof the uncorrected f vs. I plot of the swept oscillator 12 must besufficiently small so that the change in the average FM rate overadjacent time intervals does not exceed the tolerance indicated by theequation The remaining integrators 48 and 50 or many are i in the systemare activated in sequence with the n integrator correcting the rampslope after the time 1,, 1.

Hence it can be seen that the objects of the invention have beenachieved by providing a phase correction system which is easy toinstrument for wide-band chirp waveforms. The constant frequencywavefonns are made to match an ideal FM waveform at preselected pointsduring the pulse. Frequencies of the reference signals are chosen to bemultiples of a reference oscillator. Frequency of the oscillator ischosen to give the desired number of correction points during the pulse.

While in accordance with the Patent Statutes only the best knownembodiment of the invention has been illustrated and described indetail, it is to be particularly understood that the invention is notlimited thereto or thereby, but that various modifications may be madeto still fall within the purposes of the invention.

What is claimed is:

1. Apparatus to generate a precision linear frequency modulated signalwhich comprises means to generate an approximately linear frequencymodulated signal as an input signal waveform,

means to generate several constantfrequency waveforms whose frequencyand phase are made to match a pulse of an ideal frequency modulatedwavefonn at preselected points during the pulse, said means comprising agates oscillator. and input trigger actuating the oscillator, andfrequency multipliers driven by the oscillator,

means to phase-compare the input signal wavefonn with resput to theconstant-frequency waveforms at predetermined intervals to produce errorsignals, which means comprises means to selectively mix the inputsignals waveform with each respective constant-frequency waveform ptemeans to sample each mixed signal at a predetemiined time, shiftregister means actuated by the gated oscillator actuating the samplingby the gate means, means to integrate the gated signals and gate theintegrator outputs to produce the error signals, such shift regTstermeans further sequentially sending the error signals of last said gatemeans to correct the means to generate an approximately linear frequencymodulated signal as an input signal waveform.

2. Apparatus according to claim 1 where the predetermined widths forgating of the mixer output signals are chosen so that the phase shiftdoes not change by more than 45 during the sample time.

3. Apparatus according to claim 11 which includes a low-pass filterreceiving the output of one of the means to mix the input signalwaveform and the appropriate constant frequency waveform,

means to detect the zero beat-time of such mixed signal,

means to compare such detected zero beat-time to a desired zerobeat-time indicated by the shift register and generate an error simial,and

means to control the ramp slope in the generator by such error signalbefore the outputs of last said gate means are sent to the generator.

41. Apparatus according to claim 3 where the low-pass filter receivesthe output of the means to mix receiving the highest frequency constantfrequency.

5. Apparatus according to claim. l where the means to generate theapproximate linear frequency modulated signal is a swept oscillatoroperating in the microwave region, a ramp generator driving the sweptoscillator, a stalo, and a mixer combining the signal from the sweptoscillator and the stalo.

1. Apparatus to generate a precision linear frequency modulated signalwhich comprises means to generate an approximately linear frequencymodulated signal as an input signal waveform, means to generate severalconstant-frequency waveforms whose frequency and phase are made to matcha pulse of an ideal frequency modulated waveform at preselected pointsduring the pulse, said means comprising a gates oscillator, and inputtrigger actuating the oscillator, and frequency multipliers driven bythe oscillator, means to phase-compare the input signal waveform withrespect to the constant-frequency waveforms at predetermined spacedintervals to produce error signals, which means comprises means toselectively mix the input signals waveform with each respectiveconstant-frequency waveform gate means to sample each mixed signal at apredetermined time, shift register means actuated by the gatedoscillator actuating the sampling by the gate means, means to integratethe gated signals and gate the integrator outputs to produce the errorsignals, such shift register means further sequentially sending theerror signals of last said gate means to correct the means to generatean approximately linear frequency modulated signal as an input signalwaveform.
 2. Apparatus according to claim 1 where the predeterminedwidths for gating of the mixer output signals are chosen so that thephase shift does not change by more than 45* during the sample time. 3.Apparatus according to claim 1 which includes a low-pass filterreceiving the output of one of the means to mix the input signalwaveform and the appropriate constant frequency waveform, means todetect the zero beat-time of such mixed signal, means to compare suchdetected zero beat-time to a desired zero beat-time indicated by theshift register and generate an error signal, and means to control theramp slope in the generator by such error signal before the outputs oflast said gate means are sent to the generator.
 4. Apparatus accordingto claim 3 where the low-pass filter receives the output of the means tomix receiving the highest frequency constant frequency.
 5. Apparatusaccording to claim 1 where the means to generate the approximate linearfrequency modulated signal is a swept oscillator operating in themicrowave region, a ramp generator driving the swept oscillator, astalo, and a mixer combining the signal from the swept oscillator andthe stalo.